freebsd/share/man/man9/malloc.9

.\"	$NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
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.Dd June 16, 1996
.Dt MALLOC 9
.Os FreeBSD
.Sh NAME
.Nm malloc ,
.Nm MALLOC ,
.Nm free ,
.Nm FREE
.Nd kernel memory management routines
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/malloc.h>
.Ft void *
.Fn malloc "unsigned long size" "struct malloc_type *type" "int flags"
.Fn MALLOC "space" "cast" "unsigned long size" "struct malloc_type  *type" "int flags"
.Ft void
.Fn free "void *addr" "struct malloc_type *type"
.Fn FREE "void *addr" "struct malloc_type *type"
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized memory in kernel address space for an
object whose size is specified by
.Fa size .
.Fn free
releases memory at address
.Fa addr
that was previously allocated by
.Fn malloc
for re-use.
The
.Fn MALLOC
macro variant is functionally equivalent to
.Bd -literal -offset indent
(space) = (cast)malloc((u_long)(size), type, flags)
.Ed
.Pp
and the
.Fn FREE
macro variant is equivalent to
.Bd -literal -offset indent
free((addr), type)
.Ed
.Pp
Unlike its standard C library counterpart
.Pq Xr malloc 3 ,
the kernel version takes two more arguments.  The
.Fa flags
argument further qualifies
.Fn malloc No Ns 's
operational characteristics as follows:
.Bl -tag -width indent
.It Dv M_NOWAIT
Causes
.Fn malloc
to return
.Dv NULL
if the request cannot be immediately fulfilled due to resource shortage.
Otherwise,
.Fn malloc
may call sleep to wait for resources to be released by other processes.
If this flag is set,
.Fn malloc
will return 
.Dv NULL
rather then block.  Note that
.Dv M_WAITOK
is defined to be 0, meaning that blocking operation is the default.
.It Dv M_ASLEEP
Causes
.Fn malloc
to call 
.Fn asleep
if the request cannot be immediately fulfilled due to a resource shortage.
M_ASLEEP is not useful alone and should always be or'd with M_NOWAIT to allow
malloc to call
.Fn asleep
and return
.Dv NULL
immediately.  It is expected that the caller will at some point call
.Fn await
and then retry the allocation.  Depending on the routine in question, the
caller may decide to propagate the temporary failure up the call chain
and actually have some other higher level routine block on the async wait
that
.Fn malloc
queued.
.It Dv M_WAITOK
indicates that it is Ok to wait for resources.  It is unconveniently
defined as 0 so care should be taken never to compare against this value
directly or try to AND it as a flag.  The default operation is to block
until the memory allocation succeeds. 
.Fn malloc
can only return
.Dv NULL
if 
.Dv M_NOWAIT 
is specified.
.El
.Pp
The
.Fa type
argument is used to perform statistics on memory usage, and for
basic sanity checks.
The statistics can be examined by
.Sq vmstat -m .
.Pp
A
.Fa type
is defined using the
.Va malloc_type_t
typedef like this:
.Bd -literal -offset indent
/* sys/something/foo_extern.h */

extern malloc_type_t M_FOOBUF;

/* sys/something/foo_main.c */

malloc_type_t M_FOOBUF = {
	"Foo Buffers",
	"Buffers for foo data in transit to the InfImpDrive"
};

/* sys/something/foo_subr.c */

...
MALLOC(buf, struct foo_buf *, sizeof *buf, M_FOOBUF, M_NOWAIT);

.Ed
.Sh RETURN VALUES
.Fn malloc
returns a kernel virtual address that is suitably aligned for storage of
any type of object, or
.Dv NULL
if the request could not be satisfied and 
.Dv M_NOWAIT 
was set.  If 
.Dv M_ASLEEP
was set and
.Fn malloc
returns
.Dv NULL ,
it will call
.Fn asleep
as a side effect.
.Sh SEE ALSO
.Xr vmstat 8
.Sh DIAGNOSTICS
A kernel compiled with the
.Dv DIAGNOSTIC
configuration option attempts to detect memory corruption caused by
such things as writing outside the allocated area and imbalanced calls to the
.Fn malloc
and
.Fn free
functions. Failing consistency checks will cause a panic or a system console
message:
.Bl -bullet -offset indent -compact
.Pp
.It
panic:
.Dq malloc: bogus type
.It
panic:
.Dq malloc: allocation too large
.It
panic:
.Dq malloc: wrong bucket
.It
panic:
.Dq malloc: lost data
.It
panic:
.Dq free: address 0x%x out of range
.It
panic:
.Dq free: type %d out of range
.It
panic:
.Dq free: unaligned addr Aq description of object
.It
panic:
.Dq free: item modified
.It
panic:
.Dq free: multiple free[s]
.It
.Dq Data modified on freelist: Aq description of object
.El